The present invention relates to a fluid feed apparatus and method for quantitatively discharging and feeding various types of fluids such as adhesives, clean solder, grease, paints, hot melts, chemicals, and foods, in manufacturing processes of electronic components, household electrical appliances, and other fields.
Liquid dispensers have conventionally been used in various fields. With recent years' needs for downsizing and higher recording density of electronic components, there is a growing demand for a technique to control fluid materials with high accuracy and stability.
For example, in the field of surface mount technology (SMT), whereas there are trends toward faster, very-smaller, higher-density, higher-grade, and unmanned mounting techniques, problems that the dispensers have been facing can be summarized into:
1) Higher accuracy of the quantity of fluid material to be applied; PA1 2) Shorter discharging time; and PA1 3) Very-smaller quantity of material to be applied at each one-time dispensation. As the conventional liquid dispenser, such a dispenser using the air pulse method as shown in FIG. 12 has been widely used, hitherto. Its technique is introduced, for example, in "Jidoka Gijutsu (Automation Technology), '93, Vol. 25, No. 7." The dispenser using the method works in such a way that a constant amount of air supplied from a constant-pressure source is pulsatively applied into a container 150 (cylinder) so that a constant amount of fluid corresponding to an increase of the internal pressure of the container 150 is discharged. PA1 (1) Variations in the discharge amount due to discharge-pressure pulsation; PA1 (2) Variations in the discharge amount due to differences in head of fluid; and PA1 (3) Changes in the discharge amount due to changes in the viscosity of fluid.
As an alternative of the aforementioned air pulse method, a dispenser using a single-shaft eccentric pump of the rotary displacement type, which is commonly known as Moyno pump, has been put into practical use. The Moyno pump is also called a snake pump after its snake-like motion. Details of the technique are introduced, for example, in "Haikan Gijutsu (Piping Technology), July '85." FIG. 11 illustrates an example of its construction.
Reference numeral 100 denotes a main shaft (drive shaft), 101 denotes a ball bearing, 102 denotes a rotor of a snake pump, 103 denotes a stator thereof, 104 and 105 denote universal joints for coupling the rotor 102 with a coupling rod 106 and coupling the coupling rod 106 with the drive shaft 100. The rotor 102 is, so to speak, a male screw having a circular cross section, while the stator 103 as a female screw corresponding to the male screw having a hole cross section formed into an oval shape.
The rotor 102 is fitted into the stator 103. When the rotor 102 is rotated at the eccentric shaft center, the rotor 102 is put into up-and-down motion with rotation inside the stator 103. The fluid entrapped between the rotor 102 and the stator 103 is continuously fed out from suction to discharge side by an endless, limitless piston motion.
However, dispensers of these methods have had the following problems.
[1] Problem of the dispenser of the air pulse method:
The shorter the operating time and the shorter the discharging time, the more noticeably the phenomenon (1) appears. On this account, measures have been taken such as providing a stabilizing circuit for ensuring a uniform height of air pulses.
The reason of the problem (2) is that since the capacity of a void portion 152 within the cylinder differs depending on the liquid remaining level H, the degree of pressure change in the void portion 152 varies to a large extent depending on the H with a constant-amount feed of high-pressure air. With a drop of the liquid remaining level, the application amount would decrease to, for example, 50 to 60% of the maximum value, as a problem. For this reason, there have been taken measures such as detecting the liquid remaining level H for each-time discharging operation and adjusting the time duration of pulses so that a uniform discharge amount is ensured.
The problem (3) takes place, for example, when a material containing a large amount of solvent has undergone a change in viscosity with time. One of the countermeasures for this problem has been that the tendency of viscosity change with respect to the time axis is previously programmed in a computer and, for example, the pulse width is adjusted so that any influence of viscosity change is corrected.
In any of the countermeasures for the above problems, the control system including a computer becomes complex whereas there is a difficulty in managing irregular variations in environmental conditions (e.g., temperature). Thus, none of the above countermeasures has been a drastic solution.
[2] Problems of the dispenser using a snake pump:
When the snake pump is involved, the dispenser is the displacement type in which the fluid is entrapped in a closed space of constant capacity and transferred as such. Therefore, the dispenser has a constant flow rate characteristic that it is less affected by viscosity change, load change on the pump discharge side, or the like, as compared with the above-described dispenser of the air pulse method. However, in the pump of the present method, which owes its pumping action to the operation that the rotor 102 is put into a reciprocating linear motion while it is rotating within the stator 103, the rotor 102 has principally a one-sided support structure and the stator 103 serves also as a bearing that supports the rotor 102.
Accordingly, when the rotation speed of the drive shaft 100 is increased, or when the discharge-side pressure increases due to an increase in the pump load, the rotor 102, which has only a poor positioning-holding function, is likely to result in an unstable motion. As a result, the clearance between the rotor 102 and the stator 103 would vary, which in turn would cause the internal leak amount to vary, incurring a problem of deteriorated flow rate accuracy. Another serious problem would be changes with years in the flow rate characteristic due to eccentric wear of the stator 103 and the rotor 102. Therefore, when the rotor is reduced in diameter for a snake pump to be used as a dispenser, the accuracy of the total discharge flow would be at most .+-.10 to 20% on the aforementioned accounts.
Also, for example, when the dispenser is attempted to reduce the rotor diameter to D=0.5 mm .phi. or less in response to a demand for an ultra-low flow rate of the dispenser (e.g., Q=10.sup.-5 cm.sup.3 /sec or less), the conventional structure involving metal-to-metal contact between rotor and stator would undergo elastic deformation, sliding wear, damage, and the like due to a deterioration of the rotor strength because the rotor is driven by the metal-to-metal contact without any adjustment thereof. Thus, the attempt has been far from practical use.